1. Field of the Invention
This invention relates generally to brushgear for miniature motors, and more specifically to brushgear for miniature motors comprising brush arms, each constituting of a terminal strip and a commutator contactor strip for making electrical contact with a motor commutator by means of a brush mounted thereon, both being joined into one piece; the brush arms being held in position by inserting them into grooves provided on a case cover of a motor case; and arrangements being made to allow the terminal and commutator contactor strips of the brush arms to be made of desired metals and the brush arms to be firmly held in position.
2. Description of the Prior Art
The performances (for example, torque) of a DC motor changes with changes in the contact pressure of brushes against the commutator (hereinafter referred to as brush pressure). It is necessary, therefore, to obtain a motor having uniform performances by selecting the brush pressure so that the motor can be run in the optimum state and preventing the brush pressure from fluctuating. Particularly, in a miniature motor whose brush pressure is determined by the resiliency of the brushes thereof made of metal strips, as shown in FIG. 1, holding the brush arms securely in position is of critical importance.
The present inventor has proposed brushgear where brush arms are securely held in position (Japanese Utility Model Publication No. 7214/1970, and Japanese Utility Model Publication No. 7215/1970). FIG. 1 shows a crosssectional view of a miniature motor using the brushgear of the above-mentioned conventional invention. FIGS. 2 through 4 are diagrams of assistance in explaining the brushgear of the conventional invention. In the following, the brushgear of the conventional invention will be briefly described referring to FIGS. 1 through 4.
In FIG. 1, reference numeral 1 refers to a motor case; 2 to a case cover made of synthetic resin; 3 to a permanent magnet; 4 to a rotor; 5 to a rotating shaft; 6 to a commutator; 7 to brush arms; 8 to a brush holder, respectively. FIG. 2 is a right-hand side view of the case cover 2 having thereon and brushgear shown in FIG. 1. Reference numerals 2 and 6 through 8 correspond with like numerals in FIG. 1. 9 indicates a terminal strip; 10 to a commutator contactor strip; both the terminal and commutator contactor strips being bent to a substantially L-shape to form a brush arm 7. On a free end of the commutator contactor strip 10 mounted is a brush 11 made of carbon or a suitable material for making electrical contact with a commutator 6. As shown in FIG. 3A, the brush arm 7 has a bead 12 formed by embossing inward the L-shaped bent portion thereof. As shown in FIG. 3B, which is a crosssectional view taken along line A--A' in FIG. 3A, the bead 12 has a circular-arc shaped crosssection. On the brush holder 8 on the case cover 2, provided is a groove for retaining a brush arm 7 in position to bring the brush 11 in electrical contact with the commutator 6 and leading out the terminal strip 9 for external connection, as shown in FIG. 4. The groove 13 has a shape corresponding with the L-shape of the brush arm 7, as shown in FIG. 2, and the width W of the groove 13 (as shown in FIG. 4) is made slightly smaller than the height H of the bead 12 (as shown in FIG. 3B). By inserting the bead 12 in the groove 13, the brush arm 7 is securely held in position, whereby the brush 11 makes electrical contact with the commutator 6 at a stable pressure by the resiliency of the commutator contactor strip 10.
In the foregoing, the conventional type of brushgear invented by the present inventor has been described in reference to FIGS. 1 through 4. The brush arm 7 in brushgear is usually made of phosphor bronze, beryllium bronze, etc. having good electrical conductivity and high resiliency. Phosphor bronze or beryllium bronze as used for this type of brush arm 7 is suitable as a material for the commutator contactor strip 10 requiring resiliency, but not desirable as a materials for the terminal strip 9 because of brittleness or the difficulty in soldering. As a material for terminals, a flexible and electrically conductive material, such as copper or brass, is generally desirable to ensure the ease of soldering and the resistance to repeated bending. For this reason, it is conceived to use materials suitable for each of the terminal strip 9 and the commutator contactor strip 10 of the brush arm 7 and join both into an integral structure by various measures, for example, joining both (i) by upsetting rivets, or (ii) by spot welding.
The joining method as described in (i), however, is not desirable due to increases in the number of required parts and the number of manufacturing steps. The joining method of (ii), on the other hand, involves the difficulty in spot welding phosphor bronze or berylium bronze with copper or brass.